This invention relates generally to the isomerization of hydrocarbons. More specifically, the invention involves an isomerization zone and an isomerized product fractionation zone in which a stabilized effluent stream from the isomerization zone is separated into high octane product streams and low octane product streams by means of fractional distillation and by making use of a dividing wall column and a non-divided column. The stabilized isomerization zone effluent is generally comprised of hydrocarbons containing between 5 and 7 carbon atoms per molecule.
Isomerization is an important process used in the petroleum industry to increase the research octane number (RON) of light naphtha feeds. In current practice, the naphtha (C5-C10 fraction) obtained from atmospheric distillation of petroleum is separated by means of fractional distillation into light naphtha and heavy naphtha. The light naphtha is generally sent to an isomerization process unit and the heavy naphtha is generally sent to a catalytic reforming process unit. In both the isomerization process unit and the catalytic reforming process unit, the RON values of the respective naphtha fractions are improved. High RON values are a desired characteristic for naphtha streams that are sent to the gasoline pool because gasoline spark ignition engines perform better and can achieve greater fuel efficiency with higher RON gasoline.
The product streams from isomerization processes (isomerate), unlike the product streams from catalytic reforming processes (reformate) are virtually free of aromatic compounds. Low aromatic concentrations are a desired characteristic for naphtha streams that are sent to the gasoline pool because of increasingly stringent specifications for aromatics in gasoline. As a result of the increasingly stringent specifications for aromatics in gasoline, there has been growing interest in the petroleum industry in processing light naphtha in isomerization process units.
The present invention relates in particular to C5-C6 fraction light naphtha feeds to isomerization units that are rich in C5-C7 molecules. The C5-C6 fraction is generally produced through fractionation of full range naphtha in such a manner that the majority of the C7 molecules found in the full range naphtha are excluded from the C5-C6 fraction. The major portion of C7 molecules found in full range naphtha are excluded from the C5-C6 fraction that is fed to most light naphtha isomerization units because excessive cracking of C7 molecules takes place at typical isomerization reactor conditions suitable for isomerizing C5-C6 rich feeds. However, a small percentage of the C7 molecules from the full range naphtha will be included in the C5-C6 fraction as a result of overlap that is characteristic of distillation processes. Therefore, the term “C5-C6 fraction” will be used herein to designate a fraction that contains C5-C7 molecules but in practice is materially a C5-C6 fraction.
Once-through isomerization processes, or processes in which the isomerization reactor effluent is not separated into high octane and low octane streams for the purpose of recycling low octane streams to the isomerization reactor, are typically limited to a maximum isomerate product RON of about 84 with typical isomerization unit feeds. The terms “isomerization unit feed” and “light naphtha feed” are used interchangeably herein to refer to the feed stream that is supplied to the isomerization unit for processing into isomerate product. Once-through isomerization processes generally cannot achieve isomerate product RON values in excess of 84 because the isomerization reactor conversion cannot exceed the equilibrium conversion attainable with commercial isomerization catalysts under isomerization conditions.
Consequently, the separation of the isomerization reactor effluent in isomerization processes is critical to achieving desired RON targets for isomerate product that exceed a RON of 84. In order to maximize the isomerate product RON, it is desirable to separate the isomerization reactor effluent into different molecular structural classes. In general, multibranched paraffins (paraffins having two or more branches) have higher RON values than straight chain and single branched compounds. It is desirable, therefore, to separate the high octane multibranched compounds (as well as high octane isopentane) as isomerate product and recycle lower octane straight chain and single branched paraffins to the reactor feed. It is generally not desirable to recycle multibranched paraffins to the reactor feed because doing so would result in the conversion of a portion of the high octane multibranched paraffins into lower octane straight chain and single branched paraffins in the isomerization reactor.
Several methods that have been utilized to achieve the desired separation between high octane components and low octane components in isomerization reactor effluents in applications with C5-C6 fraction light naphtha feeds are described in Domergue, B., and Watripont, L. World Refining, May 2000, p. 26-30 and in Aranovich, I., Reis, E., and Shakun, A. Hydrocarbon Engineering, April 2012, p. 20-26. Each of the separation methods discussed in these two articles improves the isomerate product RON compared with the isomerate product RON that can be obtained in a once-through isomerization process.
The separation methods discussed in the Domergue and Watripont and the Aranovich, Reis, and Shakun articles that can be used to increase the isomerate product RON include: the use of a Deisopentanizer column to recover isopentane from the isomerization unit feed before the isomerization unit feed is introduced to the isomerization reactor; the use of a Deisohexanizer column to separate high octane and low octane C6 compounds from the isomerization reactor effluent; the use of a Deisopentanizer column in conjunction with a molecular sieve adsorption process; the use of a Deisohexanizer column in conjunction with a molecular sieve adsorption process; and the use of a the use of a Deisopentanizer column in conjunction with a Depentanizer column and a Deisohexanizer column.
In each configuration discussed in the Domergue and Watripont and the Aranovich, Reis, and Shakun articles, the placement of the Deisopentanizer is upstream of the isomerization zone and the placement of the Depentanizer column and the Deisohexanizer column is downstream of the isomerization zone.
The majority of the configurations discussed in the Domergue and Watripont and the Aranovich, Reis, and Shakun articles rely on separation and recycle of low octane streams to the isomerization reactor as a means to increase the isomerate product RON. A Deisohexanizer, for example, separates high octane C6 molecules (principally dimethylbutanes) from low octane C6 molecules (principally normal hexane and methylpentanes). The resulting high octane stream that is produced by the Deisohexanizer column separation is withdrawn from the isomerization process as a product stream and the low octane stream that is produced by the Deisohexanizer column separation is recycled to the isomerization reactor. A composite isomerate product RON value of approximately 88 can be achieved through the use of a configuration with solely a Deisohexanizer in the product fractionation zone.
In order to achieve composite isomerate product RON values in the range of 88 to 93, it is necessary to include low octane C5 molecules in the recycle to the isomerization reactor along with low octane C6 molecules. Since the separation and recycle of low octane C5 and C6 molecules cannot be accomplished with only a Deisohexanizer column, a more complex process scheme is required to achieve composite isomerate product RON values in the range of 88 to 93. The separation of high octane and low octane C5 and C6 molecules has traditionally been accomplished through configurations which rely on pairing distillation with a molecular adsorption process or alternately by using a complex distillation configuration in which a Deisopentanizer column is placed upstream of the isomerization reactor in a feed fractionation zone and a Depentanizer column and a Deisohexanizer column are used in an isomerized product fractionation zone downstream of the isomerization reactors. In the latter configuration, the isomerization reactor effluent is sent to a Depentanizer column in an isomerized product fractionation zone, where the Depentanizer is used to separate a C5 rich stream from the balance of the isomerization reactor effluent. A C5 rich stream is removed from the first end of the Depentanizer column and sent to a Deisopentanizer column in the feed fractionation zone, where high octane isopentane is separated from the normal pentane in the C5 rich stream (the Deisopentanizer receives a combined feed consisting of the isomerization unit feed and the C5 rich stream that is recycled from the Depentanizer column and the Deisopentanizer separates isopentane from the balance of the combined feed). Isopentane from the C5 rich stream is removed from the first end of the Deisopentanizer column as a high octane isomerate product and normal pentane from the C5 rich stream is removed from the second end of the Deisopentanizer column and recycled to the isomerization reactor. The balance of the isomerization reactor effluent which is fed to the Depentanizer (in the isomerized product fractionation zone) is removed from the second end of the Depentanizer and sent to a Deisohexanizer column to separate high octane and low octane C6 compounds. The term “first end of the column” is used herein to refer to the overhead distillate system (at the top) of the column and the term “second end of the column” is used herein to refer to the bottom of the column.
Achieving composite isomerate product RON values in the range of 88 to 93 using currently known art requires high energy inputs to separate high octane streams for removal from the isomerization process as isomerate products and low octane streams for recycling to the isomerization reactor. The most energy intensive separations are the distillation processes which separate close boiling molecules; in particular the separation of dimethyl butane from methylpentane in a Deisohexanizer and the separation of isopentane from normal pentane in a Deisopentanizer require large energy inputs to the respective distillation column reboilers to perform the desired separations.
High energy inputs may also be required in isomerization configurations which depend exclusively on a Deisohexanizer for separating high octane streams from low octane streams. High energy usage is generally required when a configuration with only a Deisohexanizer is used to separate high and low octane streams to produce composite isomerate products with RON values in the range of about 86 to 88.
None of the methods outlined in the Domergue and Watripont article or the Aranovich, Reis, and Shakun article make use of a dividing wall column to separate high octane components and low octane components in isomerization reactor effluents. In general, a significant improvement in the efficiency of separation can be achieved through separations that are performed in dividing wall columns compared with the use of multiple non-divided columns to perform the same separations because of the superior thermal efficiency of dividing wall columns.
One novel process scheme for separating high octane components and low octane components in isomerization reactor effluents in applications with C5-C6 fraction light naphtha feeds is described in U.S. Pat. No. 6,395,951. The separation scheme presented in U.S. Pat. No. 6,395,951 employs a unique configuration consisting of an adsorptive separation zone followed by a dividing wall fractionation zone to separate isomerization zone effluent streams into high octane and low octane fractions. Low octane straight chain paraffins such as normal pentane and normal hexane are removed in the absorptive separation zone for recycle to the isomerization zone and a dividing wall column in the dividing wall fractionation zone separates low octane single branched C6 paraffins from high octane multibranched C6 paraffins and from a high octane C6-C7 bottoms stream. The separation in the dividing wall column for this design is notably different than separations which are made in typical deisohexanizer column designs in that the majority of (high octane) methylcyclopentane is intentionally removed as part of the high octane C6-C7 bottoms stream. This contrasts with a typical deisohexanizer design that does not have an adsorptive separation section to remove low octane straight chain paraffins. Normal hexane (a straight chain molecule) is present in the feed to a typical deisohexanizer column that does not have an adsorptive separation section to remove low octane straight chain paraffins, and because normal hexane has a very low octane value, it is desirable to include as much normal hexane as possible in the low octane fraction containing low octane paraffins with a single branch so that the normal hexane can be recycled to the isomerization zone for conversion to isomerized products. Normal hexane and methylcyclohexane are close boiling molecules, and as a result of including the majority of normal hexane in the low octane fraction containing low octane C6 paraffins with a single branch, the majority of methylcyclopentane is also removed from a typical deisohexanizer column in the low octane stream containing normal hexane and C6 paraffins with a single branch. In effect, the methods described in U.S. Pat. No. 6,395,951 use a dividing wall column to create high octane and low octane fractions that have different compositions with respect to methylcyclopentane than typical deisohexanizer separations.
The process scheme described in U.S. Pat. No. 6,395,951, like the other process schemes discussed in the Domergue and Watripont and Aranovich, Reis, and Shakun articles for producing composite isomerate products with RON values in the range of 88 to 93, requires a large capital investment to construct, has high utility requirements, and is difficult to operate. A process scheme which reduces the utility costs, capital costs, and operating complexity of separating isomerization reactor effluent streams to produce a composite isomerate product with a RON value in the range of 88 to 93 would constitute an improvement over the current art.
The use of a fractional distillation scheme involving a dividing wall column and a non-divided column in the present invention to separate an isomerization reactor effluent in a process with a C5-C6 fraction light naphtha feed provides significant advantages versus methods that are currently publically known because the dividing wall fractional distillation process is more energy efficient, less costly to construct, and easier to operate than currently known processes. Unlike currently known methods for producing composite isomerate products with RON values in the range of 88 to 93, the present invention does not require the use of an adsorptive separation zone or a plurality of fractionation zones in which energy intensive separations are performed in each fractionation zone.